HANDLING EXCEPTIONS AND EVENTS - Java Programming: From Problem Analysis to Program Design - page 767

As shown in Figure 11-8, the GUI contains 16 buttons, a text field, and a window. The
buttons and the text field are placed in the content pane of the window. The user enters
the input using the various buttons, and the program displays the result in the text field.
To create the 16 buttons, you use 16 reference variables of type JButton , and to create
thetextfield,youuseareferencevariableoftype JTextField . You also need a
reference variable to access the content pane of the window. As we did in previous
GUI programs (in Chapters 6, 8, and 10), we create the class containing the application
program by extending the definition of the class JFrame , which also allows you to
create the necessary window to create the GUI. Thus, we use the following variables to
create the GUI components and to access the content pane of the window:

PROBLEM
ANALYSIS
AND GUI AND
ALGORITHM
DESIGN

private JTextField displayText = new JTextField(30);
private JButton[] button = new JButton[16];

Container pane = getContentPane(); // to access the content pane
As you can see from Figure 11-8, the GUI components are nicely organized. To place
the GUI components as shown in the figure, we first set the layout of the content pane
to null andthenusethemethods setSize and setLocation to place the GUI
components at various locations in the content pane. The following statement instanti-
ates the JTextField object displayText and places it in the content pane:

displayText.setSize(200, 30);
displayText.setLocation(10, 10);
pane.add(displayText);

The size of displayText is set to 200 pixels wide and 30 pixels high and it is placed
at position (10, 10) in the content pane.
To assign labels to the buttons, rather than write 16 statements, we use an array of
strings and a loop. Consider the following statement:

1
1

private String[] keys = {"7", "8", "9", "/",
"4", "5", "6", "*",
"1", "2", "3", "-",
"0", "C", "=", "+"};
Because the size of the displayText is 200 pixels wide and each row has four
JButtons , we set the width of each JButton to 50 pixels wide. To keep the height
of each JButton the same as the height of displayText , we set the height of each
JButton to 30 pixels.
The user enters input via the buttons. Therefore, each button can generate an action
event. To respond to the events generated by a button, we will create and register an
appropriate object. In Chapters 6, 8, and 10, we used the mechanism of the inner class
to create and register a listener object. In this program, we make the class containing
the application program implement the interface ActionListener . Therefore, we
only need to provide the definition of the method actionPerformed , which will be
described later in this section. Because the class containing the application program